Absorption and Gain Spectra

Ryo Hanai, P. B. Littlewood, Yoji Ohashi

We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a highly-excited exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present.

M.E. Marhic

E. H. Weinberg

Sumanta Das, Mihai Macovei

The possibility of steady-state population inversion in a small sample of strongly driven two-level emitters such as quantum dots (QDs) in microcavities, and its utilization towards amplification of light and acoustic waves, is investigated theoretically. We find that inversion and absorption spectra of photons and phonons crucially depend on the interplay between the intrinsic vacuum and phonon environments. The absorption profiles of photons and phonons show marked novel features such as gain instead of transparency and absorption reversed to gain, respectively. Furthermore, we report collectivity-induced substantial enhancement of inversion and pronounced gain in the photon and phonon absorption spectra for a subwavelength-sized QD ensemble.

Ekaterina A. Tarasevich, M. G. Gladush

The applicability of the method for measuring the probe absorption and gain spectra in a system of quantum emitters excited by a resonant driving field for studying cooperative photoluminescence is considered. The gain and attenuation of a probe signal in the system of two quantum emitters with the dipole–dipole interaction have been calculated for the conditions of observing the cooperative photoluminescence of a pair of impurity molecules described in the literature and close to them. The resulting dependences demonstrate the structure of excited collective molecular states, which can be used to reconstruct the positions of molecules in a matrix and to determine the difference in their photophysical properties. Thus, the probe absorption and gain spectra can be used to analyze the photoluminescence of quantum-entangled particles and to transform light signals.